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J. Movshon, S.P. McKee, J.T. Flynn; Binocularity, Latent Nystagmus, and the Visual Control of Smooth Eye Movements . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2660.
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Patients with early onset strabismus often exhibit a nasalward drift (latent nystagmus) during monocular viewing with either eye, and a nasalward bias in visually evoked smooth eye movements. Previously, we found that the presence or absence of binocular vision was an important component underlying the pattern of visual loss in a large sample (n= 427) of amblyopic and "at risk" patients. Here, we examine the contribution of binocularity to abnormal nasalward bias by measuring smooth pursuit and OKN in this same group, and comparing their performance to normal observers.
We recorded monocular OKN eye movements to a vertical grating moving horizontally at 9.4 deg/sec using a limbus tracker. We also recorded monocular pursuit to a 1 deg "X" target moving at 3.5 deg/sec. We used the Stereo–optical "circles" test and a binocular motion integration test to assess binocular function in the central visual field.
For normal observers, OKN and pursuit speed in the nasal direction is slightly faster than speed in the temporal direction (N/T= 1.06). In non–binocular patients, speed in the nasal direction is normal, while temporal speed is much slower (N/T = 1.24). Binocular patients have a normal N/T ratio (1.05). Predictably, most of the non–binocular patients have misaligned eyes. However, in strabismics with residual binocular function, the nasal/temporal bias is normal. Interestingly, non–binocular anisometropes also have a normal N/T bias.
These results suggest that the loss of binocular vision in early life produces the nasal bias in monocular pursuit and OKN, and is also responsible for the clinically observed latent nystagmus. These motor biases can be explained if, in non–binocular individuals, a preponderance of signals from the contralateral eye drives the visuomotor areas responsible for the generation of visually evoked smooth eye movements.
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